The invention relates to a process for preparing formylphenylboronic acids of the formula (I). 
Ortho-, meta- and para-formylphenylboronic acids are versatile building blocks in organic synthesis and are important intermediates in the synthesis of active compounds in the agrochemical and pharmaceutical industries, but the compounds are of especially high efficiency and importance as enzyme stabilizers, inhibitors and bactericides.
Despite the great commercial interest in these compounds because of the abovementioned applications, only a few, expensive routes for preparing them have been described in the literature.
Boronic acids are prepared quite generally by reacting organometallic compounds (e.g. Grignard compounds or organolithium compounds) with boron trihalides or trialkyl borates. Owing to the reactivity of the formyl group toward organometallic compounds, this procedure is only possible for preparing formylphenylboronic acids when the formyl group is protected appropriately beforehand. As raw materials, it is therefore necessary to use p-halobenzaldehydes which are, for example, acetalized and subsequently reacted to form the organometallic reagent.
Nxc3x6th et al. (Chem. Ber. 1990, 1841-1843) convert p-bromobenzaldehyde into the diethyl acetal, convert this into the corresponding Grignard reagent by means of magnesium turnings in tetrahydrofuran (THF) and, after reaction with tri-n-butyl borate, obtain formylphenylboronic acid in a yield of 70%. A disadvantage of this synthesis is the high price of bromobenzaldehyde compared to chlorobenzaldehyde, the need for ultrasonic activation in the preparation of the Grignard reagent and the use of the expensive tributyl borate; in addition, complicated purification procedures have to be employed (e.g. via the hydrolysis product 1-butanol).
Jendralla et al. (Liebigs Ann. 1995, 1253-1257) achieve an improvement to 78% by means of process improvements in the same synthetic sequence, but here, too, the abovementioned disadvantages remain.
Significantly better yields (up to 99%) were achieved by Kobayashi et al. by reaction of bromobenzaldehyde diethyl acetal with n-butyllithium and subsequent reaction with triisopropyl borate, but the high prices of bromobenzaldehyde, triisopropyl borate and butyllithium stand in the way of an economically interesting process.
There is therefore a need to develop a process for preparing formylphenylboronic acids which starts out from advantageous starting materials which are readily available commercially and makes it possible to obtain the target products in good yields and high purities by reaction with cheap boron compounds.
It was firstly established that the necessary Grignard compounds cannot be obtained from various chlorobenzaldehyde acetals by reaction with magnesium in various ethers by methods of the prior art. In the German patent application DE-A-199 60 866, which is not a prior publication, it was found that the Grignard compounds can be obtained in good yields by addition of transition metal catalysts and simultaneous mechanical activation of the magnesium. Reaction with trimethyl borate results in corresponding formylphenylboronic acids in good yields. This is an economically very interesting method of preparation which, however, requires high capital costs and places considerable demands on plant construction due to the mechanical activation of the magnesium which is required. At the same time, the products contain traces of the transition metals used in the ppm range which, depending on the application (pharmaceuticals, enzyme inhibitors), have to be removed quantitatively by costly methods.
It is therefore an object of the present invention to provide a simple and economical process for preparing formylphenylboronic acids which starts out from advantageous starting materials which are readily available commercially and makes do without the use of transition metal catalysts and without high capital costs for plant construction. At the same time, the process should give the products in very high yields and purities.
The present invention achieves these objects and provides a process for preparing formylphenylboronic acids of the formula (I) by reaction of protected chlorobenzaldehydes of the formula (II) with lithium metal in an inert solvent to form compounds of the formula (III) and subsequent reaction with a boron compound of the formula BY3 to give compounds of the formula (I) 
where Y is a straight-chain or branched C1-C6-alkoxy or C1-C5-dialkylamino group, halogen or a C1-C6-alkylthio group, and R is H or a C1-C5-alkyl or C1-C5-alkoxy radical.
The radical CHX2 is preferably an acetal of the formula (IV) or (V) 
where R1 to R4 are identical or different and are each hydrogen, C1-C12-alkyl or phenyl, or R1 and R2 together or R1 and R3 together form a 5- or 6-membered aliphatic or aromatic ring; or an oxazolidine of the formula (VI) 
or an aminal of the formula (VII) 
where R1 and R4 are as defined above and R5 and Rxe2x80x2 are each C1-C6-alkyl or aryl.
As starting compounds of the formula (II), it is possible to use protected ortho-, meta- or para-chlorobenzaldehydes.
Although the lithium metal used according to the invention is an expensive raw material on the basis of its mass, the price difference compared to magnesium on the basis of molar amounts used is comparatively small. In the present process, the metal is placed as a dispersion, powder, turnings, sand, granules, pieces, bars or in another form together with a suitable solvent in a reaction vessel and is reacted with the protected chlorobenzaldehyde. Suitable inert solvents are all solvents which react neither with the lithium metal nor with the lithiated aromatic formed under the conditions of the process of the invention, in particular aliphatic or aromatic ethers, tertiary amines or hydrocarbons, e.g. THF, diethyl ether, diisopropyl ether, di-n-butyl ether, toluene, cyclohexane or dioxane or mixtures of the inert solvents in question.
The reaction of lithium metal with protected chlorobenzaldehydes is carried out at temperatures in the range from xe2x88x92100xc2x0 C. to +35xc2x0 C., since the reaction proceeds too slowly at lower temperatures but at higher temperatures the lithium aryls formed attack and cleave the, for example, acetal, aminal or oxazolidine function. Preferred reaction temperatures are therefore in the range from xe2x88x9270 to +10xc2x0 C., particularly preferably from xe2x88x9255 to +5xc2x0 C.
The reaction of lithium with compounds of the formula (II) is generally complete after from 3 to 18 hours, in particular from 4 to 10 hours, although in some cases, depending on the nature of the protected chlorobenzaldehyde used and the solvent employed, the reaction can proceed significantly more slowly, resulting in poor space-time yields. The rate of this reaction can be increased considerably by the presence of organic redox catalysts such as biphenyl, naphthalene or other organic compounds which rapidly take up electrons from the Li metal and transfer them quickly and efficiently to the Cxe2x80x94Cl bond of the protected chlorobenzaldehyde. The redox catalysts are added in amounts of from 0 to 5 mol %.
The molar ratio of lithium to the compound of the formula (II) is usually in the range from 1.9:1 to 8:1, but larger excesses can also be used when this is, for example, advantageous for reasons of the apparatus employed, e.g. in pumped circulation apparatuses.
The concentration of the lithium compound in the solvent can be from 0.5 to 50% by weight, preferably from 5 to 35% by weight, particularly preferably from 15 to 30% by weight. The protected chlorobenzaldehyde can either be metered in or (especially in the case of relatively large Li pieces) can all be placed in the reaction vessel initially.
The reaction of the resulting organolithium compounds of the formula (III) with the boron compounds is, to achieve high selectivity, carried out at low temperatures in the range from +20 to xe2x88x92110xc2x0 C., preferably from 0 to xe2x88x9280xc2x0 C. It is possible either to add the boron compounds as liquids or solutions to the solution of the organolithium compound or to add the solution of the organolithium compound to the initially charged boron compound (which may, if desired, have been initially charged as a solution in hydrocarbons or ethers). It is advantageous to use a small excess, in particular an excess in the range from 5 to 50%, of boron compound.
Suitable boron compounds are, for example, boron halides such as BBr3, BF3, BCl3 or boric esters, e.g. trimethyl borate, triisopropyl borate or tributyl borate; it is likewise possible to use mixed haloboric esters. Amines or thio compounds of boron, for example tris(diethylamino)borane or tris(n-butylthio)borane, can likewise be used.
After the reaction mixtures have thawed to room temperature, various work-up methods are suitable, for example hydrolysis by addition of water, adjustment of the pH to a slightly acidic region (2.5-6.5), distilling off the solvents, filtration and drying of the product. For economic reasons, it is of great interest to obtain the solvent used in the lithiation in water-free form and thus reuse it directly in the lithiation; thus, it can be obtained by distillation from the reaction mixture with simultaneous addition of a relatively high-boiling solvent. For example, diethyl ether can be replaced by cyclohexane or THF can be replaced by toluene.
Before the formylphenylboronic acids are separated off from the reaction mixture, for example by filtration or centrifugation, water-soluble solvents such as THF should preferably be separated off by, for example, distillation, since otherwise the solubility of the products in water is increased and the yields are correspondingly reduced. The pH of the hydrolysis mixture is firstly in the alkaline region and is brought to a value in the range from 7.5 to 1.0 before the product is isolated or before water-soluble solvents are distilled off, in order to prevent secondary reactions such as Cannizzaro reactions. Preference is here given to pH ranges from 6.0 to 3.0, particularly preferably the pH of the free boronic acid. Filtration or centrifugation of the products is advantageously carried out at temperatures in the range from xe2x88x9210 to +75xc2x0 C. Owing to their oxidation sensitivity, the formylphenylboronic acids obtained have to be dried under protective gas and under reduced pressure and at mild temperatures, preferably in the range from 20 to 80xc2x0 C.
When all work is carried out under protective gas, the product is obtained in very pure form ( greater than 99%, HPLC a/a) and can in most cases be used further without further purification.
For specific applications, purification is necessary and can be carried out, for example, by dissolution in aqueous sodium hydroxide at 0-30xc2x0 C., extraction with toluene or another hydrocarbon or ether and subsequent precipitation by acidification. Such appropriate purification methods are described, for example, in the German patent application DE-A-10 032 017.1 which is not a prior publication.